“Pandemic” and “epidemic” are principally nouns, even though they can be used adjectivally, just like any other noun: e.g. “car horn.” (I say “used adjectivally” rather than “adjectives” because some linguists would argue that “car horn” is just a single compound noun [like German Autohupe], not an adjective plus a noun. Either way, “car” specifies and so modifies “horn.”) These words are concrete nouns that denote discrete events, however long-lasting or widespread those events may be.
“Endemic” is principally an adjective. It refers to something that is widespread among, or native to a certain population or region (“the endemic coronavirus”). The word thus modifies a concrete noun by specifying something about its prevalence. The word can be used postpositively (“The coronavirus is now endemic.”), but that doesn’t make it a noun in this sense.
One can use the word “endemic” as a noun, but only in a certain, even more limited way. “An endemic” refers (only) to an endemic organism or pathogen; it is a concrete noun. It does not mean the state or condition whereby such an organism or pathogen is widespread among, or native to a certain population or region; that is, it is not an abstract noun. Yet I have noticed that some people are now using it in just this way, saying sentences like “The coronavirus is now an endemic” to mean that there now exists a condition whereby the virus is staying with us, rather than to refer to the virus itself, which is happening now to stay with us. In other words, they mean to be describing our environment, but that isn’t the accepted usage of “endemic” as a noun: “The coronavirus is now an endemic” ought to be a description of the virus (meaning “The coronavirus is now an endemic pathogen”), not of its prevalence in our environment (it shouldn’t mean, despite certain speakers’ intentions, that “There now prevails among us an endemic pathogen”).
Some will argue the newer usage that I’m resisting is a natural and useful broadening of the use of “endemic” as a noun, occasioned precisely by the evolving real-world coronavirus event. But I think it is a potentially misleading and confusing extension, precisely because it casts something as an event that it should not. Pandemic and epidemic refer to concrete temporal events, but this new usage of endemic would be referring to an atemporal, abstract condition that some nevertheless will easily mistake (or perhaps are already mistaking) for a concrete temporal event because of the morphological and semantic parallels with the other two terms. Better, in my view, to use endemic solely as an adjective, or at the very least to restrict its use as a noun to the well-established, concrete sense of an endemic organism or pathogen, not its prevalence.
This isn’t solely hard-line, fuddy-duddy prescriptivism (even if it is that, in part). What’s at stake is how we organize our emerging new understanding of the coronavirus from a global public health event that will (implicitly) have an end into a condition that is (at least potentially) as everlasting as the common cold or the flu, and how we speak about our understanding inevitably shapes the very ideas we’re conceiving. Clear speech means clear thinking.